1. Field of the Invention
The present invention relates to the conversion of lignocellulosic feedstock by hydrolysis to obtain simple sugars, such as glucose and xylose.
2. Description of the Prior Art
It is known that lignocellulosic feedstock, such as particulate wood in the form of chips, sawdust and shavings, can be converted by acid hydrolysis to produce simple sugars, such as glucose and xylose, which can then be fermented to produce ethanol and many other fuels or chemicals. In developing commercial acid hydrolysis processes, problems have been encountered due to the low sugar yields caused by the degradation of the sugar and low sugar concentration in the product stream, the corrosive nature of the acid, and the difficulties of conveying solids into and out of a pressurized hydrolysis reactor.
Chemically, the cell wall tissue of wood is a complex mixture of polymers. These polymers are classified into two groups, the polysaccharides and lignin. The polysaccharides of wood are collectively known as holocellulose, which means total cellulosic carbohydrates. The holocellulose accounts for about 70 to 80 percent of the extractive-free woody tissue, with lignin comprising the remainder.
The holocellulose is composed of cellulose and a mixture of other polysaccharides, collectively known as hemicelluloses.
Cellulose is a high molecular weight linear polymer composed of glucose anhydride units. Hemicellulose is a mixture of shorter chain polymers of the anhydrides of xylose, arabinose, glucose, mannose, and galactose, with xylan and galactoglucomannan as the most prevalent species.
Lignin is a complex polymer of condensed phenylpropane units, and functions as the adhesive material of wood, joining together the fibers and other cells to form the firm anatomical structure of wood.
The major chemical reactions in the acid hydrolysis of lignocellulose can be represented by the following reactions of cellulose and xylan: ##STR1## The cellulose reaction is characteristic of the reactions of the six carbon sugar components (cellulose, and the glucose, galactose, and mannose fractions of hemicellulose) while the xylan reaction is characteristic of the reactions of the five carbon fraction of the hemicellulose (xylose and arabinose).
Glucose, galactose and mannose are yeast fermentable sugars, whereas the pentoses, such as xylose and arabinose are nonfermentable.
Hemicelluloses hydrolyze substantially more easily and rapidly than cellulose. For example, temperatures and acid concentrations that require a few hours to hydrolyze cellulose to glucose, can readily convert much of the hemicellulose into simple sugars in a matter of minutes.
Under conditions where hydrolysis occurs, the sugars that form will undergo decomposition in the presence of the acid, with the pentoses decomposing more rapidly than the hexoses. Varying the conditions of acid hydrolysis changes the rate of the hydrolysis and degradation reactions, and causes variations in the yields of the various sugar products.
The typical mechanism of a dilute acid hydrolysis process involves contacting wood particles, which can be in the form of chips, shavings or sawdust, with a heated dilute acid solution in a pressurized reaction chamber. The dilute acid is generally an inorganic acid, such as sulfuric, hydrochloric, phosphoric, nitric, or hydrofluoric, with sulfuric being preferred. This results in a solid phase reaction between the acid and the particulate wood, which yields the desired glucose product suspended in the dilute acid solution. As the desired products are the sugars, it is important to stop the reactions before the sugars can decompose to hydroxymethylfurfural (HMF), furfural, and other degradation products.
One known method for converting particulate wood to glucose is by means of plug-flow hydrolysis, which comprises premixing particulate wood with a dilute acid solution, followed by passing the mixture through a reactor at elevated temperature and pressure. In plug-flow hydrolysis, the particulate wood chips and dilute acid solution remain in the reactor for the same amount of time. In order to minimize degradation, the reaction is conducted at very high temperatures, on the order of about 200.degree.-260.degree. C., wherein the sugar formation reaction proceeds at a faster rate than the degradation reaction. However, practical drawbacks limit the yield of this process to about 50-60%. Also, the sugars produced are quite dilute due to the low yields and to the difficulty of pumping concentrated slurries. In general, the movement of particulate wood, especially under pressure, is a mechanically complex process, and is often the most difficult and expensive step involved.
In another method, called "percolation hydrolysis," a dilute acid solution is passed through a reactor chamber packed with particulate wood. The wood remains in the reactor long enough for complete hydrolysis to occur, but the water and acid flows through the reactor with a much shorter residence time. Thus, the sugars diffuse from the wood into the liquid phase and are washed out of the reactor and cooled, before substantial degradation can occur.
The cooling of the hydrolyzate and the quenching of the reactions can best be accomplished by passing the acid stream through a flash valve and into a flash tank, which brings about rapid cooling. At temperatures at or below about 120.degree.-140.degree. C., the sugar degradation reactions do not occur at an appreciable rate. The sugar-water-acid stream is then neutralized, and the sugar undergoes fermentation to the final product by conventional means known in the art.
A disadvantage of percolation hydrolysis is that very large amounts of dilute acid solution are necessary to wash the sugars quickly from the reactor and thus, the concentration of sugar product in the dilute acid solution can be quite low. The advantage of the process is that it is relatively simple proven technology. All solids handling is carried out at ambient temperature and atmospheric pressure. Percolation reactors were developed in Germany in the 1920's and 1940's, and are used extensively in the U.S.S.R.
Another method proposed for producing sugars from particulate wood is by means of counter-current hydrolysis. In counter-current hydrolysis, a flow of dilute acid solution contacts a body of particulate wood which is moving in a direction opposite to the flow of the dilute acid solution. The counter-current flow of the dilute acid solution and the particulate wood results in a much higher yield of sugars from the wood, minimal degradation, and a relatively high concentration of glucose in the dilute acid solution.
The primary disadvantage of counter-current hydrolysis is the extreme mechanical complexity and expense of moving the solids and liquids in opposite directions, the difficulty of achieving good liquid-solid contact, and the inability to impose a temperature profile in a single vessel. In theory, the counter-current reactor is the most efficient type of hydrolysis reactor, but no practical counter-current reactor designs have yet been demonstrated.